Vannette: How carbon dioxide can trap heat -- and lead to a warmer planet

Matthew Vannette

Published 8:13 am, Tuesday, May 23, 2017

To the editor:

In a letter to the editor dated April 22, Richard Heiny of Midland questioned the existence of a physical mechanism by which a transparent gas can trap heat. To address his question, I offer this explanation of the physics leading to the greenhouse gas effect.

From about 1860-1880 the British physicist John Tyndall conducted experiments on the ability of gases to absorb and emit “radiant energy,” or infrared light. Infrared light has a wavelength that is longer than we can see with our eyes. There is a related property called frequency, and frequency times wavelength gives the wave speed. In this case, the speed of light. We cannot see infrared light, but it will heat up a wire. Tyndall used a thermopile, an instrument to measure the heating effect, to determine how much infrared light passed through different gases including water vapor, carbon dioxide, and methane. What he found was that each gas absorbs light of different wavelengths and with different absorbing strengths.

It is true that at visible wavelengths carbon dioxide is transparent, but it strongly absorbs infrared light. Light of the right wavelength (with its associated frequency) is not reflected. When infrared light is absorbed by a carbon dioxide molecule, the molecule vibrates – the oxygen atoms bounce like they are on springs with a frequency that matches the light absorbed. We say the vibrating molecule is “excited.” When the molecule changes from the excited to a non-excited state, we say it “relaxes” by “emitting” light. The wavelength and frequency of the light emitted generally matches the wavelength and frequency of the light absorbed. Each type of molecule absorbs light of different frequencies depending on what atoms are needed to make it. This effect allows us to study the atmospheres of planets we’ve never been to.

The question is how does this lead to a warming effect? Imagine a bit of light radiated off the Earth’s surface. If that light escapes to space, it takes its energy with it, cooling the Earth. However, if a molecule absorbs the light, that energy does not necessarily escape. When the molecule relaxes it emits the light in a random direction. There is an approximately 50 percent chance the emitted energy will go back toward the Earth, keeping things warmer here. Thus, about half of the light absorbed by a single carbon dioxide molecule in the atmosphere is trapped on Earth. A single molecule can do this many, many times, and it will continue to do so as long as it is in the atmosphere.

The same basic mechanism works for all the other greenhouse gases. In fact, the effect is more pronounced for water and methane because they have many more wavelengths they are excited by. This is the physical effect – absorption and emission. It is not reflection in the strictest sense, but that analogy is often used.

Geologically, temperature increase has been tied to an increase in global carbon dioxide, typically over a time frame of thousands of years. We have seen a tremendous rise in global carbon dioxide over the last century. One may argue that 400 parts per million is insufficient to affect a system. It is, after all, only 1 molecule out of every 2,500. I implore you to consider what 25 parts per billion (1 out of every 40 million molecules) of lead in Flint’s drinking water caused. Concentration is only part of the issue. The effect of the molecule is also important. The science describing the greenhouse effect has been studied for more than 150 years. Claiming the small concentration of greenhouse gas is insufficient to affect such a large system ignores how effective the molecule is at changing the properties of the system.

We can discuss alternative ideas for how, as a society, to address the effects of a warmer planet. But, confusion about the mechanism is a very different beast. I hope this offers some clarity on the physics so we can move toward discussing actions.